Medical images such as traditional X-ray images are often taken on sheets of photographic film, but CT (X-ray computed tomography), MRI (magnetic resonance imaging) and ultrasound images are most often displayed on a screen, since such images are computed images resulting from series of measurements. With the advent of digital X-ray detectors, radiology departments are making the transition to film-less hospitals. X-ray images are no longer taken and stored on film, but captured and stored digitally.
In many medical imaging techniques, but in particularly in X-ray, important anatomical features in the image can have very low contrast, compared to the overall image. This is due to the nature of the X-ray projection, in which all structures along the X-ray trajectory (i.e. from the X-ray tube to the X-ray film) are overlapped. To conserve the small contrasts, medical images from X-ray (but also from MRI, CT and other modalities) are captured and stored at up to 16 bit resolution.
Typically, the screens on which the images are viewed are capable only of 8 bit to 10 bits gray scale resolution. This is one of the reasons why traditional film, which can render high gray scale resolution, is still used in hospitals. The human eye is limited to approximately 1000 so-called just noticeable differences, corresponding to 10 bits. To fully exploit the high dynamic range of the data, a number of user interface strategies have been developed to benefit of the 14 bit gray scale on an 8 to 10 bit gray scale display. Such known methods include manually selecting a limited interval or window of gray scale values and expanding the values in the selected interval to the full gray scale capacity of the screen, whereby a dark interval and a light interval are clipped on the displayed image. Most common methods are based on mouse and keyboard interaction to adjust gamma or histogram distributions.
The problem with the methods used so far to visualize high bit depth on relatively low contrast display is that they do not encourage a natural exploration of the image material. Often the professional will suspect or know that a predetermined region of the image is of interest and it is desired to a locally enhance the contrast in the region of interest (ROI).
The invention provides a method and a system for manipulating an image displayed on a screen, where a movable object such as a hand or a finger of an operator is positioned relative to the screen, i.e. in front of the screen without touching the screen. The spatial position of the movable object relative to the screen is detected, i.e. its x, y, and z coordinates. A region of interest (ROI) on the screen including the two coordinates (x, y) is selected, and a property (e.g. contrast) of the image in the selected region of interest is changed in dependence on the distance (z) of the movable object from the screen.
Using e.g. a finger, the operator can point at the region of interest on the screen without touching the screen, and the system will then first detect the x and y coordinates of the finger, i.e. the point on the screen closest to the finger, and select a region around that point. Depending on the distance of the finger from the screen the contrast (or other image property) will change.
This is a more natural user interface for this particular problem. The interaction modality of the invention that responds to pointing to identify the region of interest and additionally provides input to determine the degree of local contrast enhancement is of great benefit.
The invention provides the use of touchless interaction (cross capacitance sensing, CCS) to create a local contrast enhancement dependent on hand or finger position in x, y and z position relative to the display. The x, y data is used to determine the center of the region in which the local contrast enhancement is made as well as the mean value. The z (zoom) position is used to determine the size of the region and the degree of enhancement.
According to the invention it is proposed to adjust the bitmap gray level to display gray level mapping (loosely called gamma curve) in a region of interest on the display. This adjustment will spread gray levels around an identified mean value. The position of the region and the value of the mean level are identified from x, y position coordinates and the size of the region as well as the degree of spread (the gamma) is determined by the z position. The invention can be used with any input system that provides x, y, z data but it is particularly advantageous to use the touchless cross capacitance sensing for local contrast enhancement, which results in an effective natural interaction and will be of great benefit particularly in medical applications. Touchless interaction based on cross capacitance sensing provides just such a modality. The cross capacitance sensing technology is described e.g. in WO 01/103621 and WO 03/010486.
In FIG. 1 is shown a system according to the invention with a controller 10 connected to a screen 11. The controller comprises circuits including conventional hardware and software for displaying an image on the screen 11. The controller preferably receives the image as digital data. The image can be a monochrome (black-and-white) or a color image in any suitable data format. The controller outputs control signals to the screen 11 for displaying the image.
In connection with the screen there is a device for detecting the spatial position of a movable object relative to the screen. In the preferred embodiment this device comprises a set of electrodes 12a, 12b, 12c and 12d arranged at the edges of the screen. In the shown embodiment the electrodes 12a, 12b, 12c and 12d are arranged at respective corners of the screen. One or more of the electrodes are arranged as transmitters, and one or more of the remaining electrodes are arranges as receivers. An electrical signal such as 100 kHz, 3 Volt peak-to-peak is imposed on one or more of the transmitting electrodes, whereby a corresponding electric field is generated in the space in front of the screen.
When an operator of the system introduces a movable object such as a finger or a hand 13 into the electric field in front of the screen, the object will disturb the electric field, and the disturbance can be detected by the receiving electrodes. This is referred to as cross capacitance sensing. By properly arranging the electrodes relative to the screen, in particular the number of electrodes and their positions, and properly choosing the electric signals such as waveform, frequency, amplitude, sequential use of transmitting and receiving electrodes etc., it will be possible to detect not only the spatial 3-D position of the movable object relative to the screen but also movements and complex gestures of the hand 13 of an operator.
When e.g. a monochrome X-ray image is displayed on the screen, the operator may often wish to manipulate the image and enhance the image contrast in a region of interest. This is done as follows and as defined in the flow chart in FIG. 3. The operator first approaches a finger or other movable object to the region of interest ROI in the image displayed on the screen. When the finger enters the electric field the space in front of the screen, this will be detected by the controller. The controller measures x, y, z simultaneously but the method uses the x, y coordinates first to determine a mean value around which contrast enhancement should take place. This can either be the value of the pixel with coordinates x, y at which the user is actually pointing or a suitable average of over pixels in its vicinity. The method then uses the z value, i.e. the distance of the finger from the screen, to establish the amount of contrast enhancement to be applied. Typically this enhancement will be zero if z is above a certain threshold value, i.e. outside the space with the electrical field, and maximum below a threshold value, i.e. close to the screen.
Similarly the region will either be constant or maximal for z above a certain threshold value and constant or minimal for z below a threshold value.
The method then determines the amount of clipping and decides whether to reduce the region so that the amount of clipping can be reduced. The contrast enhancement is then applied and the image displayed. This is a dynamic process that is repeated as long as the presence of the movable object is detected in the electric field in front of the screen or is otherwise interrupted.
FIG. 4 illustrates the contrast enhancement. Shown on the horizontal axis is the input gray level of the digital image, i.e. bitmap, which will typically have a higher bit resolution than the display supports. On the vertical axis is plotted the output gray level of the display. For simplicity the normal contrast curve is assumed to be linear, and the effect of display gamma curve is ignored, which is an independent and additional mapping that takes place either before or after the mapping described here. FIG. 4 illustrates how the input to output mapping is enhanced around a ‘mean’ value by a contrast enhancement factor. The enhancement maps a smaller input range to a larger output range, but means that some input gray values are clipped to black or white output values. Preferably, these clipped input gray level values occur outside the region of interest, so that the entire region of interest will be contrast enhanced. It is possible to monitor how many pixels are clipped inside the region of interest and reduce the region of interest to minimize this. Another possibility is to use a smooth S-curve instead of the linear curve shown in FIG. 4.
The region of interest can have a predefined shape such as an ellipse as illustrated in FIG. 1 and a predefined size. However, by detecting the movements of the operator's finger, the operator may use his finger to “draw” the contour of an individual region of interest on the screen, or he may be given the choice between several standardized shapes and sizes.
Likewise, the operator may also be given the choice of image parameter to be changed, such as brightness whereby the bit map window on the horizontal axis is moved to the left or to the right.
The invention is described with reference to cross capacitance sensing of the movable object used for interacting with the screen. However, other contact free methods may also be used, such as optical sensing.
The invention is ideal for manipulating medical images but the invention may also be used for image processing in general.